1. Field of the Invention
The present invention relates to a master disc and the recording of video information thereon. More particularly, the invention concerns such disc and such recording wherein the master disc is flexible.
2. Description Relative to the Prior Art
In order to record video information at a relatively high packing density, the video information is commonly recorded in the form of micron and sub-micron sized data bits such as pits, bumps, apertures, etc. Focussing a recording beam to such small dimensions calls for the use of a high numerical aperture optical system. Attendant with the use of a high numerical aperture optical system is an extremely small depth of focus. As an example, if the recorded video information is in the form of data bits as small as half a micron in size, the maximum allowable depth of focus can be as small as one-fourth of a micron, depending upon the wavelength of the recording beam. Maintaining this depth of focus is difficult even in a static system, i.e., one in which the optical system and record medium are not moving. The difficulty, as might be expected, is multiplied in a dynamic recording system wherein the record medium and/or the optical system are in motion.
In a typical video disc recording system, a twelve inch master disc spins at a high rate of speed, commonly 1800 rpm. During recording, a modulated recording beam scans the master disc so that billions of video data bits are recorded along a spiral track. All during this recording process the recording beam must be kept in proper focus, which means that the spinning disc surface must be kept within the depth of focus of the recording beam. As stated, the depth of focus for such a system is extremely small and it is very difficult to maintain proper focus.
U.S. Pat. No. 3,825,323 discloses a disc recording system wherein the disc has a surface coated with a conductive film. An electrode is mounted to the lens system which focusses the recording beam on the disc. The electrode and conductive film thus form a capacitor, the capacitance of which varies as a function of lens to disc spacing. The capacitance can thus be monitored to provide an indication of focus. A servomechanism adjusts the lens to disc spacing in response to the monitored capacitance to keep the recording beam in focus on the disc. Such a system apparently works best with a disc of glass, for example, which is optically ground and polished to a high degree of optical flatness. Some localized surface irregularities may not, however, be detected because the sensed capacitance of a perfectly flat disc can be identical to the sensed capacitance of a disc having several closely spaced surface depressions and humps which cancel each other from a capacitance standpoint, resulting in low quality recording on the disc due to focus error. Even if a small, localized surface irregularity were detected as a change in capacitance, the lens positioning adjustment may have to take place so fast that the required compensation is beyond the response time of the servomechanism, especially if the lens system is relatively heavy.
U.S. Pat. No. 4,006,294 discloses a recording head assembly which is spaced a nominal distance from a flexible video disc mounted to a turntable. The head assembly contains a lens system for focussing a recording beam (i.e., a laser) on the surface of the video disc. A small aperture is provided in the head assembly to direct an air stream toward the video disc. A change in the spacing between the head assembly and the disc causes a corresponding change in the back pressure of the air stream. The head assembly, by means of a pair of expandable chambers connected to each other by a fluid restrictor, senses the change in back pressure and automatically adjusts the height of the lens system from the disc surface to restore proper focus.
Another type of focus control device for use with video discs which utilizes air flow is disclosed in U.S. Pat. No. 4,003,059. In that patent, a lens system is mounted on a floating device provided with "floating shoes". As the video disc spins during recording, air flow between the spinning disc and the floating shoes produces a high enough air pressure on the shoes to float the floating device, and the lens system mounted thereto, at a constant height above the disc surface, thereby maintaining proper focus.
Both of the "air flow" type recording systems discussed above depend upon the video disc to provide a flat surface, preferably an optically flat surface, to float or locate against in order to define a focal plane. While the back pressure sensitive recording system of U.S. Pat. No. 4,006,294 is disclosed for use with a flexible video disc laying on top of a turntable, it is doubtful that such an arrangement would perform satisfactorily in those applications wherein it is required to record sub-micron sized data bits, where the depth of focus can be as small as a quarter micron. In particular, if the flexible disc is slightly stretched or warped it will not lay flat on the turntable, thus necessitating extensive and rapid focus adjustments of the lens system. In actual practice, the lens system may literally vibrate in an effort to maintain proper focus. As might be expected, such a condition is inherently unstable and can lead to unsatisfactory and non-uniform results. Further, even if the flexible video disc were perfectly flat, air pockets may be trapped under the disc when the disc is placed on the turntable, or even during the recording process itself. Any such air pockets may cause difficulties in focussing, such as those described above. Likewise, using a flexible video disc with the "floating head" recording system of U.S. Pat. No. 4,003,059 may lead to similar problems, especially when recording video data bits of sub-micron dimensions with a lens system having an extremely small depth of focus.
It is thus seen that while several recording systems are available for recording video information on a video disc at high packing densities, these systems generally require the use of a rigid, optically flat master disc. An optically flat video disc is expensive to manufacture, however, since a glass (or quartz, etc.) blank must be ground and polished to optical dimensions. As a practical matter, these expensive glass discs are easily damaged unless handled with great care. These problems are not insurmountable from a commercial standpoint when it is desired to mass produce replicate discs from a single master disc. The high initial cost of the master disc can be absorbed by allocating it over a large number of replicate discs. The situation is different, however, when no or only a few replicate discs are to be produced. Allocating the high cost of the master disc over a small number of replicate discs makes each replicate disc relatively expensive.